Add an option to disable decimal

This patch replaces the existing --with-system-libmpdec option with a
--with-libmpdec={system,builtin,none} option, which allows to tell
Python whether we want to use the system libmpdec (already installed),
the libmpdec builtin the Python sources, or no libmpdec at all.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
---

Index: b/configure.ac
===================================================================
--- a/configure.ac
+++ b/configure.ac
@@ -2385,13 +2385,20 @@
 AC_MSG_RESULT($with_system_ffi)
 
 # Check for use of the system libmpdec library
-AC_MSG_CHECKING(for --with-system-libmpdec)
-AC_ARG_WITH(system_libmpdec,
-            AS_HELP_STRING([--with-system-libmpdec], [build _decimal module using an installed libmpdec library]),
+AC_MSG_CHECKING(for --with-libmpdec)
+AC_ARG_WITH(libmpdec,
+            AS_HELP_STRING([--with-libmpdec], [select which libmpdec version to use: system, builtin, none]),
             [],
-            [with_system_libmpdec="no"])
+            [with_libmpdec="builtin"])
 
-AC_MSG_RESULT($with_system_libmpdec)
+AC_MSG_RESULT($with_libmpdec)
+if test "$with_libmpdec" != "none"; then
+   MPDEC=yes
+else
+   DISABLED_EXTENSIONS="${DISABLED_EXTENSIONS} _decimal"
+   MPDEC=no
+fi
+AC_SUBST(MPDEC)
 
 # Check for support for loadable sqlite extensions
 AC_MSG_CHECKING(for --enable-loadable-sqlite-extensions)
Index: b/setup.py
===================================================================
--- a/setup.py
+++ b/setup.py
@@ -1972,7 +1972,7 @@
     def _decimal_ext(self):
         extra_compile_args = []
         undef_macros = []
-        if '--with-system-libmpdec' in sysconfig.get_config_var("CONFIG_ARGS"):
+        if '--with-libmpdec=system' in sysconfig.get_config_var("CONFIG_ARGS"):
             include_dirs = []
             libraries = [':libmpdec.so.2']
             sources = ['_decimal/_decimal.c']